Latex additive for water-based drilling fluids

ABSTRACT

The use of polymer latices is described for fluid loss control in water-based drilling fluids. The latices used are water insoluble. The latices are essentially non-swelling in an aqueous solution. The latices are selected from known latices such that they are absorbed within a filter cake building up at the interface between the wellbore and porous formations in essentially the same state as they are in the aqueous drilling fluid during circulation.

This invention relates to a latex additive for wellbore drilling fluids.More specifically, it pertains to an additive for reducing the loss ofdrilling fluid into the formations surrounding the wellbore.

BACKGROUND OF THE INVENTION

For the production of hydrocarbon wells, boreholes are drilled intosubterranean formations. Following standard procedures, a fluid iscirculated during drilling from the surface through the interior of thedrill string and the annulus between drill string and formation. Thedrilling fluid, also referred to as “drilling mud”, is used toaccomplish a number of interrelated functions. These functions are:

(1) The fluid must suspend and transport solid particles to the surfacefor screening out and disposal;

(2) It must transport a clay or other substance capable of adhering toand coating the uncased borehole surface, both (a) to exclude unwantedfluids which may be encountered, such as brines, thereby preventing themfrom mixing with and degrading the rheological profile of the drillingmud, as well as (b) to prevent the loss of downhole pressure from fluidloss should the borehole traverse an interval of porous formationmaterial;

(3) It must keep suspended an additive weighting agent (to increasespecific gravity of the mud), generally barites (a barium sulfate ore,ground to a fine particular size), so that the entire column of drillingfluid is not interrupted upon encountering pressurized pockets ofcombustible gas, which otherwise would tend to reduce downhole pressure,as well as creating a “blowout” in which the fluid and even the drillstem are violently ejected from the well, with resulting catastrophicdamages, particularly from fires;

(4) It must constantly lubricate the drill bit so as to promote drillingefficiency and retard bit wear.

The industry distinguishes between largely three classes of drillingfluids: oil-based, water-based and so-called synthetic muds. Whereasoil-based muds are recognized for their superior qualities for most ofthe drilling operations themselves, they become increasing undesirabledue to their impact on the environment and stricter environmentallegislation. Water-based muds are expected to replace oil-based mud asthe drilling fluid of choice in major geographical areas.

A drilling fluid typically contains a number of additives. Thoseadditives impart desired properties to the fluid, such as viscosity ordensity. One class of additives is used as fluid loss agents to preventthe drilling fluid from entering into porous formations.

The basic mechanism of fluid loss control is generally the formation ofa filter cake at the interface of the porous or permeable formationlayers. As part of the drilling fluid is forced into the formation bythe higher pressure within the wellbore, larger particles and additivesare left behind and accumulate at the face of the formation. The filtercake thus formed can be regarded as a membrane that protects theformation from further invasion of wellbore fluids. Fluid-loss controlagents are selected in view of their quality to form a competent filtercake.

Known examples of such fluid-loss control agents are water-solublepolymeric additives added to the drilling fluid to improve the sealingof the filter cake. These fluid-loss polymers are most commonly modifiedcelluloses, starches, or other polysaccharide derivatives and aresubject to temperature limitations. In particular, most start to failaround 105-120 degrees C.

Latices on the other hand are described for example in the U.S. Pat. No.5,770,760 using latex to thicken water-based drilling fluids. The latexis added to the mud and chemically treated to produce the functionalpolymer that is in a solubilized form.

The use of latices for the purpose of fluid loss control is describedfor example in the U.S. Pat. Nos. 4,600,515 and 4,385,155. In thoseapplications, however, polymer latices are used in a water-soluble form.

It is therefore an object of the present invention to provide a novelclass of fluid loss agents for drilling fluids.

SUMMARY OF THE INVENTION

The invention comprises the use of polymer latices for fluid losscontrol in water-based drilling fluids. The latices used are waterinsoluble. Preferably, the latices are essentially non-swelling in anaqueous solution.

The latices are selected from known latices such that they are absorbedwithin a filter cake building up at the interface between the wellboreand porous formations in essentially the same state as they are in theaqueous drilling fluid. Hence the latices used for this application arenot coagulated or further crosslinked.

Another selection criterion for suitable latices is that the Tg, orglass transition temperature of the polymer must be lower than thetemperature of the drilling application so that the polymer is in arubbery or fluid state. In this state the polymer particles aredeformable which improves the sealing characteristics of the filtercake.

The polymer latices can be of any water insoluble polymers, copolymersor terpolymers, for example synthesized by emulsion polymerization. Themain chemical types can be summarized as:

Polymers and copolymers in which the principal repeat units are derivedfrom monoolefinically-unsaturated monomers such as vinyl acetate, vinylesters of other fatty acids, esters of acrylic and methacrylic acids,acrylonitrile, styrene, vinyl chloride, vinylidene chloride,tetrafluoroethylene and related monomers.

Polymers and copolymers in which the major proportion of the repeatunits are derived from 1,3-dienes such as 1,3-butadiene (butadiene)2-methyl-1,3-butadiene (isoprene) and 2-chloro-1,3-butadiene(chloroprene), with smaller proportions of the repeat units beingderived from the monoolefinically unsaturated monomers such as styreneand acrylonitrile, or others of category 1.

Other polymers such as polyisobutenes containing minor amounts ofcopolymerised isoprene, polyurethanes and other monomer units.

Latices used for the purpose of the present invention include but arenot restricted to styrene-butadiene copolymer latex (SBR), andstyrene-acrylate-methacrylate terpolymer latex (SA).

Compatibility with other solids present in the drilling fluids mayrequire the use of an additional stabilizer as additive to the waterbased drilling fluid. This may be the case for certain types of SBRlatices. SA latices appear stable at ambient and moderate temperatures(to ca. 60C) but become destabilized at elevated temperatures. Otherlatex chemistries may be more stable. The stabilizer is generally addedat a dosage of 10% of the latex concentration or less. Care must betaken in selection to minimize formation damage from free stabilizer.The most effective stabilizers are anionic surfactants typified bysodium docdecyl sulphate (SDS), Aerosol OT (AOT), and polymericstabilizers/surfactant such as NPE (a 30% aqueous solution of ammoniumsalt of sulfated ethoxylated nonylphenols). Nonionic surfactants such asthe Triton series, an octylphenol polyether alcohol with varying numbersof ether linkages per molecule, commercially available from UnionCarbide. Synperonics can also be used to stabilize the latex.

Further additives as known in the art may be added to impart otherdesired properties to the mud system. Such known additives includeviscosifying agents, filtrate reducing agents, and weight adjustingagents. Other preferred additives are shale-swelling inhibitors, such assalts, glycol-, silicate- or phosphate-based agents, or any combinationthereof.

These and other features of the invention, preferred embodiments andvariants thereof, possible applications and advantages will becomeappreciated and understood by those skilled in the art from thefollowing detailed description and drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 compares polymeric and latex fluid loss additives showing thecumulative Fluid loss at 30 minutes at 25C as a function of applieddifferential pressure;

FIG. 2 compares the performance of agglomerated and non-agglomerated SBRlatices showing the cumulative fluid loss at 30 minutes at 25C as afunction of applied pressure;

FIG. 3 shows the cumulative fluid loss at 30 minutes at 25C as afunction of applied pressure for SA and SBR latices;

FIG. 4 illustrates the effect of Temperature on performance of glassypolymer comparing the filtration performance from SA type latex at 25and 80C (above and below Tg=59C); and

FIG. 5 illustrates the effect of temperature on fluid loss in latexsystems using a barite/Xanthan composition as base fluid.

MODE(S) FOR CARRYING OUT THE INVENTION

Several different water insoluble latices were tested for their use asfluid loss additives.

Firstly, their filtration properties were examined using a ½ area APIHTHP filter press as function of temperature and pressure. Typicallypressures in the range 100-500 psi and temperatures of 25C to 150C wereused. The cumulative fluid loss after 30 minutes was used tocharacterize the filtration performance.

A lightly weighted polymer based fluid, consisting of 4 g/l Xanthan gum(IDVIS), 160 g/l API barite, adjusted to pH 8 with NaOH was used as thebase system for these tests. FIG. 1 compares the filtration performanceat 25C of a stabilized SBR latex, with a conventional fluid losspolymer: polyanionic cellulose (PAC). The latex, coded LPF5356, is astyrene-butadiene latex with a Tg of ˜−20C and commercially available asPliolite LPF5356 from GOODYEAR. The latex is polydisperse with aparticle size range of 100-600nm. The latex slurry was added to the baseformulation at 3.5% or 7% active with nonionic surfactant Triton X405 at10% of the latex concentration. The PAC was added to the control systemat a concentration of 5 g/l. The graph shows much lower fluid loss forthe latex than for the conventional polymeric additive, with much lesspressure dependence. The latex particle improves sealing in the filtercake.

Similar results were obtained with LPF7528 of GOODYEAR's Plioliteseries, having an average particle size of 150 nm. FIG. 2 comparesfiltration performance of LPF7528 to LPF5356 at 25C both stabilized withionic surfactant SDS, again at 10% of latex concentration.

Further examples, coded LS1 and LS2, are styrene-acrylate-methacrylatelatices of GOODYEAR's Pliotec range of commercial latices with a size of˜150 nm diameter which by varying the ratio of the different monomersvary in Tg between 59 and 0, respectively. At ambient temperature allare stable with respect to other solids and need no additionalstabilisers.

FIG. 3 summarizes their performance added as 3.5% active to the bariteweighted base system. The latex LS2 having a lower than ambient Tgperforms well. LS1 that has a Tg of 59C performs badly. In its glassystate the particle does not deform to pack well within the filter cake.If the test is repeated above its Tg, at 80C, it performs in similarfashion to the other latices, see FIG. 4. As the latex LS1 wasdestabilized at elevated temperature, surfactant SDS was added to theformulation at a concentration of 0.35% (10% of the latexconcentration).

It is generally found that fluid loss increases with increasingtemperature. In addition the polymeric additives will degrade at hightemperatures. FIG. 5 shows the effect of temperature on various latexsystems in the barite weighted Xanthan gum base fluid. The base systemshows rapid loss of filtration control by 80C. In general, the latexsystems show a much smaller increase in fluid loss over this range. Thehigh Tg latex LS1 shows improved fluid loss at elevated temperatures.Other system limitations appear at higher temperatures, in particularthe nonionic surfactant is no longer an effective stabilizer above 105C,resulting in flocculation of the latex It was found that the ionicsurfactants, and the ionic polymer D135 continued to stabilize thepolymers above this temperature. A further problem occurred with theXanthan gum that also begins to lose performance at around 105-110Ccausing barite sag. Scleroglucan biopolymer is stable to highertemperatures. 8 g/l scleroglucan (Biovis) / 160 g/l API barite basedsystems containing SA and SBR latex with various stabilizers were hotrolled at 120C overnight (16 h). HPHT fluid loss was measured at 120Cbefore and after aging. Table 1 summarizes the results. No barite sagwas observed in any of the systems. In this system the latice latex isslightly less affected than the SBR latex as is expected from theirrelative performance at temperature.

TABLE 1 30 minute HPHT fluid loss after aging overnight (16 h) at 120°C. Fluid loss System Before ageing After ageing LPF7528 + SDS 8.8 12.8LPF7528 + AOT 9.2 10 LPF7528 + NPE 7.6 8.4 LS1 + SDS 6.4 7.2 LS1 + AOT10.6 10 LS1 + NPE 6.4 7.2 Base System 20.8 240

The examples given so far are for fresh water systems. The latices arealso stable to added salt. Tests performed in the presence of 5% KCl orNaCl show no difference from results shown above. The latex is alsostable in 25% CaCl₂ brine.

Additional test were performed to evaluate formation damages caused bythe novel additives. The test method used has been described by L JFraser, P Reid, D Williamson, and F Enriquez Jr in: “Mechanisticinvestigation of the formation damaging characteristics of mixed metalhydroxide drill-in fluids and comparison with polymer-base fluids”. SPE30501. SPE Annual Technical Conference and Exhibition, Dallas, Tex. USA,Oct. 22-25, 1995.

Following the described method, a 25.4 mm diameter, 30 mm long Clashachsandstone core was presaturated under vacuum with a synthetic connatewater formulation, given in table 2.

TABLE 2 composition of connate water Salt Concentration g/Litre NaCl56.369 CaCl₂.2H₂O 6.027 MgCl₂.6H₂O 2.46 KCl 1.137 NaHCO₃ 1.332 CH₃COOH0.244

Permeability to kerosene was determined at residual water saturation.100 pore volumes of kerosene (˜350 g) were flooded through the core atthe maximum pressure used in the test, 10 psi. Then the flow rate wasdetermined for 3 applied pressures: 10, 5 and 2 psi. The core was thenmounted in a filter cell and exposed to drilling fluid for 4 hours at300 psi differential pressure, the filtration direction being oppositedirection to permeability flow. After filtration the level ofpermeability damage was determined. To quantify damage, first clean uptests were performed by flowing kerosene through at 2, 5 and 10 psi,waiting until equilibrium flow rates were achieved before stepping up tothe next pressure. These equilibrium flow rates were compared to theinitial flow rates at these pressures. Then after clean up at 10 psi,the three point permeability was again determined and a % retainedpermeability was calculated from the difference between the final andinitial permeabilities, % Kf/Ki.

Formation damage tests were performed on carbonate weighted drillingfluids. The base system was 8 g/l scleroglucan (Biovis) and 360 g/lcarbonate Idcarb 150. pH was adjusted to 9 with NaOH. To this were addedfluid loss additives: either PAC at 5 g/l or latices LPF7528 or LS1 at3.5% active, with various stabilizers: surfactants SDS, AOT, and thepolymeric stabilizer NPE at 10% of the latex concentration. Tests werecarried out at ambient temperature and at 120C. Table 3 summarizesperformance.

TABLE 3 Formation Damage tests on Clashach core. % Retained permeabilityand clean up after exposure to drilling fluid. 4h filtration, ΔP300 psiat 25° C. and 120° C. Initial Return Fluid Loss Filtration TemperatureFluid Loss Permeability Permeability % Clean-up Additive ° C. g mD % 2psi 5 psi 10 psi LPF7528 + NPE 25 2.66 583 56 7 69 65 LPF7528 + AOT 250.62 599 61 18 38 60 PAC 25 5.28 570 62 18 8 60 LPF7528 + AOT 120 9.83690 76 3 56 73 LS1 + NPE 120 5.27 817 65 22 39 61 LS1 + SDS 120 5.56 70790 5 57 85 LS1 + AOT 120 5.25 556 69 30 56 66 PAC 120 18.86 714 49 3.524 49

The latex combinations show much improved fluid loss over theconventional PAC polymer. In particular, it can be clearly seen fromtable 3 that the PAC filtration performance is significantly degraded at120C, whereas the latex formulations remain effective. The SBR latexgives similar permeability damage to PAC at room temperature, andimproves at elevated temperature. The SA type latices are very lowdamaging, particularly in combination with the anionic surfactant SDS,where return permeabilities are ˜90%. The polyanionic stabilizer NPE isslightly more damaging than the surfactant stabilizers. The ease ofclean up should also be noted, with the SA latices achieving high cleanup at low pressure. In most cases the SA latex cake cleanly detachedfrom the core face. The SBR latex cakes were more dispersive tending topinhole, as do filter cakes formulated with conventional polymers.

What is claimed is:
 1. A method for drilling a borehole wherein anaqueous drilling fluid is circulated within said borehole whiledrilling, comprising circulating in said borehole with said aqueousdrilling fluid an effective amount of an additive consisting of a latexcompound which is essentially insoluble and non-swellable in water, theglass transition temperature of the latex compound being lower than thetemperature of the drilling fluid within said borehole.
 2. The method ofclaim 1 wherein the latex is serving as fluid loss agent.
 3. The methodof claim 1, further comprising the step of letting the latex form atleast part of a filter cake within the borehole.
 4. The method of claim1 wherein the latex is added as a polymer suspension to the drillingfluid.
 5. The method of claim 1 wherein the latex is added as a polymersuspension to the drilling fluid with a given particle size or particlesize distribution and essentially maintains said particle size orparticle size distribution within the drilling fluid and as depositwithin a filter cake.
 6. The method of claim 1 wherein the latex isadded as a polymer suspension to the drilling fluid and is partiallydeposited so as to form a part of a filter cake essentially withoutundergoing further agglomeration, coagulation, crosslinking or waterinduced swelling.
 7. The method of claim 1 wherein up to 20 volume percent of latex suspension are added to said aqueous drilling fluid. 8.The method of claim 1 wherein said aqueous composition further includesviscosifying additives.
 9. The method of claim 1 wherein said aqueouscomposition further includes sufficient suspended, firmly divided solidsto form a filter cake on the wall of said borehole.
 10. The method ofclaim 9 wherein said finely divided solids include clayey material. 11.The method of claim 1 further comprising the steps of: preparing theaqueous drilling fluid; pumping said fluid through a tubular structurewith a drill bit at a bottom end; and returning said fluid through anannulus between the tubular structure and the wall of the borehole tothe surface.
 12. The method of claim 1 wherein the latex compound has aglass transition temperature in the range from −20 to 59° C.
 13. Acomposition useful as a drilling fluid in drilling a boreholecomprising: an aqueous carrier; an effective amount of an additiveconsisting of an essentially insoluble and non-swellable latex compound,the latex compound having a glass transition temperature in the rangefrom −20 to 59° C.; and sufficient suspended, finely divided solids toform together with said latex a filter cake on the wall of saidborehole.
 14. The composition of claim 13 wherein said compositioncomprises up to 20 weight per cent of said latex compound.